Oil pump

ABSTRACT

Certain preferred embodiments of the present invention provide an oil pump which is both light as well as compact. Accordingly, certain presently preferred embodiments disclose an oil pump having a housing comprising aluminum, and at least one mobile molded part therein. The mobile molded part being at least partially made from a sinterable composition comprising at least one austenitic iron-based alloy powder. The mobile molded part also has a heat expansion coefficient, which amounts to at least 60% of the heat expansion coefficient for the oil pump housing.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of PCT/EP2004/004702 filed May 4,2004, which claims priority to German Application Number 103 21 521.2filed May 14, 2003, each of which is incorporated herein by reference inits entirety.

FIELD OF THE INVENTION

The present invention relates to molded parts, methods for producingsuch molded parts, and oil pumps having a housing and at least one suchmolded part, movably arranged in said housing.

BACKGROUND OF THE INVENTION

Oil pumps of the type mentioned at the outset are particularly used ininternal combustion engines, in which higher temperatures are dominant.Conventional oil pumps according to prior art are provided with ahousing made from cast iron and mobile molded parts arranged therein,for example a toothed set of rotors in the case of an internal gearpump.

Conventional mobile molded parts are usually made from a ferriferousalloy via a sintered metallurgical production method. These mobilemolded parts are typically made from an iron-copper alloy. The use ofcast iron and iron-copper alloys results in such pumps having arelatively high weight. Considering the general trend in automobileconstruction to reduce the weight of motor vehicles it is thereforedesirable to provide oil pumps which have a lesser weight.

For this purpose, it is known from prior art to produce the oil pumphousing from cast aluminum alloy rather than from cast iron. Yet even inthese circumstances, iron-copper alloys are used for mobile molded partssuch as, for example, a set of toothed rotors arranged inside thehousing of the oil pump. However gap leakage is problematic in such oilpumps, particularly during operation in an internal combustion enginebecause of environmental temperature variations and generally hightemperatures. This problem is usually addressed by increasing thedimensions of the oil pump itself. However, increasing the pumpdimension is counterproductive since the desired weight savings areoffset by the increased size of the oil pump.

Therefore, there is a need in the art for oil pumps having both lowerweight and minimal gap leakage.

SUMMARY OF THE INVENTION

The present invention relates to molded parts, methods for producingsuch molded parts, and oil pumps having a housing and at least one suchmolded part, movably arranged in said housing. Certain preferredembodiments of the present invention include an oil pump having ahousing, made from a material comprising aluminum, and mobile moldedparts arranged inside the housing. For example, the oil pump may be anexternal gear pump with involute toothing, an interior gear pump withtrochoidal toothing and crescent, G-rotors with cycloidal gear,P-rotors, or a vane cell pump. The oil pump mobile molded parts are atleast partially made from a material that can be sintered, comprising atleast one austenitic iron-based alloy powder. Molded parts, according tocertain aspects of the present invention, also have a heat expansioncoefficient which amounts to at least 60% of the oil pump housing heatexpansion coefficient. Preferably, the molded part heat expansioncoefficient amounts to at least 70%, more preferably at least 74%, inreference to that of the oil pump housing.

Sintered molded parts, according to certain aspects of the presentinvention, may be made entirely from a material that can be sintered.Sintered molded parts may also include compound molded parts in whichthe body portion of such compound molded parts, for example, are madefrom an aluminum-containing powder mixture. Still further, such a bodyportion, further connected to another body portion, may be made fromanother sinterable material comprising at least one austeniticiron-based alloy. The body portion made of an aluminum-containing powdermixture may also be replaced by one made from solid cast aluminum.Conversely, the sinterable material molded part can comprise, forexample, body portions having a sintered layer, comprising at least onesinterable austenitic iron-based alloy powder, at the facial sides orits surface. The body portion, for example, may be made from sinteredsteel, cast steel, sintered cast iron, cast iron, or combinationsthereof.

The oil pump, according to the certain preferred aspects of the presentinvention, provides minimal gap leakage during operation in an internalcombustion engine. Therefore, larger dimensions are not necessary and,by the use of aluminum, the oil pump housing weight is considerablyreduced. The housing of the oil pump can either be made in a castingprocess or via sinter-metallurgical production methods. The housing ispreferably cast from an aluminum alloy.

The material that can be sintered, from which the mobile molded partsarranged inside the housing of the oil pump are made, are preferablyproduced from a single austenitic iron-based alloy powder. However,mixtures of several austenitic iron-based alloy powders may also beused. Furthermore, the material that can be sintered can also comprisecommon lubricants, compression agents, gliding agents, and the like.Lubricants, which are added in an amount of approximately 0.2% toapproximately 5% by weight in reference to the total amount of thematerial that can be sintered, can be self-lubricating agents, such asMoS₂, WS₂, BN, MnS, as well as graphite and/or other carbonmodifications such as coke, polarized graphite, and the like, whichprovide the mobile formed parts with self-lubricating characteristics.Binders and/or lubricants can be selected from materials of a groupcomprising polyvinyl acetate, wax, in particular amide wax such asethylene-bisstearoylamide, shellac, polyalkylene oxide and/orpolyglycol. Polyalkylene oxide and/or polyglycol is preferably used inthe form of polymer and/or copolymer with a medium molar weight in arange from approximately 100 to 50,000 g/mol, preferably approximately1,000 to 6,500 g/mol. Binders and/or lubricants are preferably used inan amount ranging from approximately 0.01 to 12% by weight, preferablyranging from approximately 0.5 to 5% by weight in reference to the totalamount of the used material that can be sintered.

Particularly suitable as austenitic iron-based alloys powders are thealloys 316L, 305, 308, 317 L and 321 or mixtures thereof. Preferredaustenitic iron-based alloy powders comprise iron and 0.005 to 0.04% byweight carbon; 0.1 to 1.5% by weight silicon; 8 to 18% by weight nickel;0 to 25% by weight chromium; 1 to 4% by weight molybdenum; and 0.05 to1% by weight manganese, in reference to the total amount of austeniticiron-based alloy.

The oil pump mobile molded parts of the present invention exhibit aBrinell-hardenss, according to DIN EN 24498-1, of at least 100 HB,preferably 120 HB, more preferred at least 130 HB, still more preferredat least 140 HB. Here, the Brinell-hardness is determined via a hardenedsteel ball used as an inserted body having a diameter of 2.5 cm and aweight of 62.5 kg. Using such hard mobile molded parts can achieve along life for the oil pumps according to the present invention. It mustbe considered that the heat expansion coefficient of conventionalaluminum cast alloys range from approximately 20 to 24 ppm, while theheat expansion coefficient of preferred sinterable austenitic iron-basedalloys used can be below that of conventional aluminum cast alloys.

Preferably, the heat expansion coefficient of the mobile molded parts,produced from the material that can be sintered ranges fromapproximately 12 to approximately 21 ppm, preferably from 16-19 ppm.Using such mobile molded parts it is ensured that the oil pump accordingto the invention fulfills the mechanical tribological requirements, inparticular for use in an internal combustion engine. Preferably, themobile molded parts of the oil pump comprise a set of rotors, with theaxial play (i.e., distance) between at least one rotor of the set ofrotors arranged on a shaft and the wall of the oil pump housing, againstwhich the rotors operates, preferably amounting to less than 50 μm,preferably less than 40 μm. Therefore, the oil pump, as a presentlypreferred embodiment of the invention, can advantageously be constructedin a very compact manner. Furthermore, the axial play defined in theabove-described manner achieves high performance of the oil pump inaccordance with certain preferred aspects of the present invention.

Certain embodiments of the invention further relate to molded parts,which are arranged inside the housing of the oil pump, and to a methodsfor producing such molded parts. In a presently preferred embodiment:

-   -   a material that can be sintered and comprising at least one        austenitic iron-based alloy, being filled into a pressed mold;    -   in a second step, a green body being pressed with a pressure of        at least 500 MPa, having a density according to DIN ISO 2738        amounting to at least 6.5 g/cm³; and    -   in a third step, the green body being sintered at a temperature        of at least 1,000° C. in a gaseous atmosphere, comprising        nitrogen and/or hydrogen.        When a gaseous atmosphere mixed from hydrogen and nitrogen is        used, the ratio of the portions of nitrogen and hydrogen is at        least 66:33, preferably more than 95:5.

Without being limited by theory, it is believed that nitride phases arecreated in the mobile molded parts made in accordance with the presentinvention using at least one austenitic iron-based alloy. It is furtherbelieved that these nitride phases fulfill the requirements necessaryfor the operation of the oil pump, made in accordance with certainpreferred aspects of the current invention, regarding hardness andstructural integrity. These properties are particular useful in internalcombustion engines. Alternative to facilitating nitride phase formationvia a mixed gaseous atmosphere according to the preferred embodiments ofthe present invention, plasma-nitration may also be performed incombination with other steps of the present method to provide fornitride phase formation.

-   -   The process according to the present invention, may further        preferably including:    -   in a fourth step, the sintered molded part is re-pressed at a        pressure of at least 600 MPa, preferably at least 750 MPa, to a        density of at least 6.7 g/cm³, according to DIN ISO 2738.

BRIEF DESCRIPTION OF THE DRAWINGS

These and additional advantages of the invention will be explained usingthe following example and the FIGURE. It shows:

FIG. 1 is a cross-section of the schematic representation of an oil pumpaccording to the invention (section).

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

FIG. 1 shows an oil pump of the type P-rotor according to the invention,in its entirety indicated with the reference number 1, (as it isdisclosed in DE 196 46 359 C2, for example, having a housing 2, which isembodied in two parts (2′, 2″) here. In this housing 2, a set of toothedrotors 4 is arranged, having an interior rotor 6 arranged on a shaft 5in the direction of the axis Z, and the planet wheels 8 encircling it.It will be appreciated by one skilled in the art that the descriptiongiven herein with respect to those FIG. 1 is for exemplary purposes onlyand is not intended in any way to limit the scope of the presentinvention.

Furthermore, the set of toothed rotors 4 comprises a rotating circularbearing 9 with bearing pockets, not shown here, in which the pivotallysupported planet wheels 8 are arranged. The interior rotor 6 issupported eccentrically in reference to the circular bearing 9, and isprovided with an approximately star-shaped exterior contour, which isprovided with external teeth. The set of toothed gears 4 comprises, asit is common, a suction area not shown in greater detail, a pressurearea, and a displacement chamber. Via the drive shaft 5, a drive momentis transferred to the toothed interior rotor 6. The mobile molded partsin the sense of the present invention, according to FIG. 1, include thecircular bearing 9, the interior rotor 6, the planet rotors 8, as wellas the shaft 5 including the catch (not shown) arranged on it.

Between the interior wall 3 of the housing part 2′ of the oil pump 1 andthe face 10 of the interior rotor 6 an axial play A is present, whichamounts to 40 μm.

The set of toothed rotors, shown in FIG. 1, is made from a sinterablecomposition comprising 1% by weight of the lubricant Licowax C, CompanyClariant GmbH, Frankfurt, which represents a polyamide wax, and 99% byweight of the austenitic iron-based alloy 316 L, comprising 0.02% byweight carbon, 0.8% by weight silicon, 13% by weight nickel, 17% byweight chromium, 2.2% by weight molybdenum, and 0.2% by weightmanganese, with the remaining portion formed by iron. The austeniticiron-based alloy 316L has been supplied by the company Hoeganaes AB,Stockholm, Sweden.

In an embodiment of the present invention, the above-defined mixture wasfirst pressed into a green body at a pressure of 600 MPa and roomtemperature, to a density in a range from 6.6 to 6.7 g/cm³, whichsubsequently in a second step was sintered in a walking beam furnace for15 minutes at a temperature of 1,280° C. under a mixed gaseousatmosphere comprising 70% nitrogen and 30% hydrogen. Subsequently, inanother step, the such sintered set of rotors was re-pressed under apressure of 800 MPa to a density of 6.8 to 7.0 g/cm3. The hardness ofmobile molded parts of the set of toothed rotors made in such manner was141 HB, 62.5/2.5 according to DIN EN 24498-1 (Brinell-hardness). Theheat expansion coefficient, determined according to DIN 51045(temperature range 25° C.-200° C.), was determined to be 17 ppm.

The set of toothed rotors made in such manner was used in a castaluminum housing made from GD-AlSi₃Cu₃ (material code 3.2163.05), whichis provided with a heat expansion coefficient of 23 ppm according to DIN51405 (temperature range 25° C.-200° C.)

The oil pump made in this fashion showed minimal gap leakage, even afterextended operation under load and increased temperatures, as common ininternal combustion engines. In reference to oil pumps according toprior art, it is considerably lighter.

According to the present invention it is also possible to produce notonly a complete set of toothed rotors, as described above, from amaterial comprising at least one austenitic iron-based alloy powder, butaccording to certain aspects of the present invention, it is alsopossible to produce the interior rotor in its entirety from a materialcomprising such austenitic iron-based alloy powder. Individual rotorcomponents may also be produced as a compound material, in particularthe interior wheel, with either the gears, for example of the interiorrotor, and/or the face 10 of the interior rotor facing the wall 3, beingproduced with a coating made from a material comprising an austeniticiron-based alloy. Furthermore, inversely, only the toothing of rotorparts of a set of rotors can be made from a different material, forexample an aluminum-based alloy, while the body material of therespective rotors being made from an austenitic iron-based alloy.Furthermore, it is also possible to merely produce the catch of theinterior rotors of a set of geared rotors from an austenitic iron-basedalloy powder material. However, the object of the present invention isnot limited to the combinations mentioned, but includes any possibleother combination of the mobile molded parts arranged in an oil pump.

Thus, certain embodiments of the present invention provide bothcompactly constructed as well as light oil pumps, which achieve anoperational life comparable to oil pumps provided with cast ironhousings. Certain preferred aspects of the present invention having beendisclosed in connection with the foregoing variations and examples,additional variations will now be apparent to persons skilled in theart. The invention is not intended to be limited to the variations andexamples specifically mentioned, and accordingly reference should bemade to the appended claims to assess the spirit and scope of theinvention in which exclusive rights are claimed.

1-7. (canceled)
 8. An oil pump comprising: a housing comprisingaluminum, said housing having a first heat expansion coefficient; and atleast one molded part comprising at least one sinterable composition,said sinterable composition comprising at least one austeniticiron-based alloy powder, and said molded part further having a secondheat expansion coefficient at least 60% of said first heat expansioncoefficient.
 9. The oil pump according to claim 8, wherein said secondheat expansion coefficient is at least 70% of said first heat expansioncoefficient.
 10. The oil pump according to claim 8, wherein said secondheat expansion coefficient is at least 74% of said first heat expansioncoefficient.
 11. The oil pump according to claim 8, wherein said atleast one molded part further comprises aluminum-containing alloypowder.
 12. The oil pump according to claim 8, wherein said austeniticiron-based alloy powder comprises: iron; and 0.005 to 0.04 weightpercent carbon, 0.1 to 1.5 weight percent silicon, 8 to 18 weightpercent nickel, 0 to 25 weight percent chromium, 1 to 4 weight percentmolybdenum, and 0.05 to 1 weight percent manganese, based on the weightof the austenitic iron-based alloy powder.
 13. The oil pump according toclaim 8, wherein said at least one molded part further comprises castaluminum, cast iron, sintered cast iron, steel, or combinations thereof.14. The sinterable composition of claim 8, further comprising from 0.2to 5 weight percent of at least one lubricant, based on the weight ofsaid sinterable composition.
 15. The sinterable composition of claim 14,wherein said at least one lubricant is MoS₂, WS₂, BN, MnS, or carbon.16. The sinterable composition of claim 8, further comprising polyvinylacetate, an amide wax, or combinations thereof.
 17. The sinterablecomposition of claim 16, wherein the amide wax comprisesethylene-bisstearoylamide, shellac, polyalkylene oxide, polyglycol, orcombinations thereof.
 18. The oil pump according to claim 8, whereinsaid at least one molded part has a hardness of at least 100 HB,according to DIN EN 24 498-1.
 19. The oil pump according to claim 8,wherein said at least one molded part has a hardness of at least 120 HB,according to DIN EN 24 498-1.
 20. The oil pump according to claim 8,wherein said at least one molded part has a hardness of at least 130 HB,according to DIN EN 24 498-1.
 21. The oil pump according to claim 8,wherein said at least one molded part has a hardness of at least 140 HB,according to DIN EN 24 498-1.
 22. The oil pump according to claim 8,wherein said second heat expansion coefficient is from about 15 to about21 ppm.
 23. The oil pump according to claim 8, wherein said second heatexpansion coefficient is from about 16 to about 19 ppm.
 24. The oil pumpaccording to claim 8, wherein said at least one molded part comprises atleast one rotor arranged on a shaft, and the axial distance between saidrotor and said housing is less than 50 μm.
 25. The oil pump according toclaim 8, wherein said at least one molded part comprises at least onerotor arranged on a shaft, and the axial distance between said rotor andsaid housing is less than 40 μm.
 26. A method for producing at least onemolded part, comprising: providing a sinterable composition, having atleast one austenitic iron-based alloy powder, to a mold; pressing saidsinterable composition under a pressure of at least 500 MPa to obtain adensity of at least 6.5 g/cm³, according to DIN ISO 2738; and sinteringsaid sinterable composition at a temperature of at least 1,000° C. in agaseous atmosphere comprising at least one of nitrogen and hydrogen. 27.The method of claim 26, wherein the ratio of said nitrogen to saidhydrogen is at least 66:33.
 28. The method of claim 26, wherein theratio of said nitrogen to said hydrogen is at least 95:5.
 29. The methodaccording to claim 26, further comprising: pressing said sinterablecomposition at a pressure of at least 600 MPa to a density of at least6.7 g/cm³, said density being in accordance with DIN ISO
 2738. 30. Themethod according to claim 26, further comprising: pressing saidsinterable composition at a pressure of at least 750 MPa to a density ofat least 6.7 g/cm³, said density being in accordance with DIN ISO 2738.31. The method of claim 26, wherein said sinterable composition furthercomprises aluminum-containing alloy powder.
 32. The method of claim 26,wherein said austenitic iron-based alloy powder comprises: iron; and0.005 to 0.04 weight percent carbon, 0.1 to
 1. weight percent silicon, 8to 18 weight percent nickel, 0 to 25 weight percent chromium, 1 to 4weight percent molybdenum, and 0.05 to 1 weight percent manganese, basedon the weight of the austenitic iron-based alloy powder.
 33. The methodof claim 26, wherein said sinterable composition further comprises from0.2 to 5 weight percent of at least one lubricant, based on the weightof said sinterable composition.
 34. The method of claim 33, wherein saidat least one lubricant is MoS₂, WS₂, BN, MnS, or carbon.
 35. The methodof claim 26, wherein said sinterable composition further comprisespolyvinyl acetate, an amide wax, or combinations thereof.
 36. The methodof claim 35, wherein the amide wax comprises ethylene-bisstearoylamide,shellac, polyalkylene oxide, polyglycol, or combinations thereof.
 37. Amolded part made by the method of claim
 26. 38. A molded part made bythe method of claim
 29. 39. An oil pump comprising: a housing comprisingaluminum, said housing having a first heat expansion coefficient; and atleast one molded part comprising at least one sinterable austeniticiron-based alloy, said alloy having a second heat expansion coefficientat least 60% of said first heat expansion coefficient; and said at leastone molded part being made by the method comprising providing asinterable composition, comprising at least one austenitic iron-basedalloy, to a mold; pressing said sinterable composition under a pressureof at least 500 MPa with a density, according to DIN ISO 2738, amountsto at least 6.5 g/cm³, and sintering said sinterable composition at atemperature of at least 1,000° C. in a gaseous atmosphere comprising atleast one of nitrogen and hydrogen.
 40. The oil pump according to claim39, wherein said at least one molded part has a hardness of at least 100HB, according to DIN EN 24 498-1.
 41. The oil pump according to claim39, wherein said at least one molded part has a hardness of at least 120HB, according to 3 EN 24 498-1.
 42. The oil pump according to claim 39,wherein said at least one molded part has a hardness of at least 130 HB,according to DIN EN 24 498-1.
 43. The oil pump according to claim 39,wherein said at least one molded part has a hardness of at least 140 HB,according to DIN EN 24 498-1.
 44. The oil pump according to claim 39,wherein said second heat expansion coefficient is at least 70% of saidfirst heat expansion coefficient.
 45. The oil pump according to claim39, wherein said second heat expansion coefficient is at least 74% ofsaid first heat expansion coefficient.
 46. The oil pump according toclaim 39, wherein said second heat expansion coefficient is from about15 to about 21 ppm.
 47. The oil pump according to claim 39, wherein saidat least one molded part comprises at least one rotor arranged on ashaft, and the axial distance between said rotor and said housing isless than 40 μm.
 48. The oil pump according to claim 39, wherein said atleast one molded part comprises at least one rotor arranged on a shaft,and the axial distance between said rotor and said housing is less than50 μm.
 49. The method according to claim 39, further comprising:pressing said sinterable composition at a pressure of at least 600 MPato a density of at least 6.7 g/cm³, said density being in accordancewith DIN ISO
 2738. 50. The method according to claim 39, furthercomprising: pressing said sinterable composition at a pressure of atleast 750 MPa to a density of at least 6.7 g/cm³, said density being inaccordance with DIN ISO
 2738. 51. The method of claim 39, wherein theratio of said nitrogen to said hydrogen is at least 66:33.
 52. Themethod of claim 39, wherein the ratio of said nitrogen to said hydrogenis at least 95:5.